Radio Frequency Identification (RFID) is a wireless data-collection technology that is becoming very popular in many services industry, logistics, manufacturing, security. The RFID system is composed by at least one interrogator (or reader) and one transponder (or tag) both interchanging information in a wireless mode. The transponder is generally fixed or tied to an object to be detected or to be monitored. The interrogator acts as base station, interacting with the transponder in order to read or write information.
In the case of passive UHF RFID transponders, the interrogator transfers energy wirelessly to the transponder by sending Radio Frequency (RF) power that the transponder may profit to obtain DC power to operate and respond at certain frequency using the backscattering modulation technique. The passive UHF RFID transponder is equivalent to an antenna loaded by a chip (integrated circuit). One of the tasks that the chip may perform is a rectification, function in which the chip obtains energy to operate as transponder. A rectifier then converts the Alternating Current (AC) that came in the RF waves sent by the interrogator into Direct Current (DC) allowing the chip to realize its rest of duties as transponder. The architecture of the rectifier is generally based on a Cockcroft-Walton circuit with two or more diode-based voltage doubler stages. The diode determines the nonlinear behavior of the chip. Another task of the chip is to generate a reply signal to the interrogator using the backscattering modulation technique. This task is also performed by nonlinear components.
One will note that, in the present application, the expression “passive RFID transponder” also designates a RFID transponder that uses the backscattering modulation technique in order to communicate with the interrogator, but in which the chip is powered by another source of energy such as a battery incorporated in the tag or a photovoltaic device. Such transponders are also called “battery assisted passive tags”, or “semi-passive tags”, and correspond to class-3 transponders according to the EPC global standardization organization. In this type of passive RFID transponders, the chip does not need to comprise a rectifier for obtaining DC power from the wireless signal emitted by the interrogator.
The nonlinear behavior of the chip generates backscattered harmonics. Until now, the presence of these backscattered harmonics is not exploited i.e. the interrogator and the transponder are designed to process only the fundamental frequency and not harmonics.